Up to now, nearly all the tooth profiles used in automobile transmission train are involute. In order to improve the load capacity of the gear pair, X-gear pairs and optimization in parameters are widely used. However, involute tooth profile has its inherent defects: undercut and large profile curvature when the tooth profile approaches the base circle or base cone for a gear with less number of teeth, thus further improvements in bending and contact strength are limited. For the bevel gear used in differentials, the tooth break caused by insufficient bending strength is the main style of the fail in gearing, so peoples always prefer to choose a gear pair with less number of teeth and larger module in order to get a higher load capacity. Limited by involute tooth profile, it is difficult to design a pair of gears with the sum of the number of teeth in the gears being less than 22, which limits a further improvement in gear strength.
The most commonly used non-involute tooth profile is cycloid profile. Although the undercut problem is eliminated and a transmission with less number of teeth can be realized, its bending strength is rather poor, and the contact strength near the pitch line is not good, being very sensitive to mounting errors, so the cycloid profile is now limited to the transmission of movement such as in watches and clocks, cannot be used for power transmission.